Photosynthesis

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Photosynthesis

• Chapter 7

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Photosynthetic Organisms

• Photosynthesis
• Process that captures solar energy
• Transforms solar energy into the chemical energy
• Chemical energy is stored in the form of a
  carbohydrate
• All organisms use organic molecules produced by
  photosynthesizers as a source of chemical energy

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Photosynthetic Organisms
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Photosynthesis

• Photosynthesis occurs in the green portions of
  plants, particularly leaves
• Leaf of flowering plant contains mesophyll tissue
  which contains cells specialized to carry on
  photosynthesis
• CO2 enters leaf
• through stomata
• Diffuses into
• chloroplasts in
• mesophyll cells

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Flowering Plants

• CO2 and water diffuse into chloroplasts
• Double membrane surrounds fluid (stroma)
• Inner membrane system within stroma form
  flattened sacs called thylakoids
• Thylakoids stacked to form grana
• Chlorophyll and
• other pigments within
• thylakoid
• membranes are
• capable of
• absorbing solar
• energy

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Photosynthetic Pigments

• Pigments molecules that absorb light at
  different wavelengths
• Most pigments absorb only some wavelengths of
  light and reflect or transmit the other
  wavelengths
• Absorption Spectra wavelength of light a
  pigment can absorb
• Organic molecules and processes within organisms
  are chemically adapted to visible light

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Photosynthetic Pigments
Absorption spectrum
Action Spectrum
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Photosynthetic Reaction

• Net Reaction
• Solar energy CO2 H2O ? (CH2O) O2

Reduction
Oxidation
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Photosynthesis Overview
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Photosynthetic Reaction

• Light Reaction - Chlorophyll absorbs solar energy
  and energizes electrons
• Electrons move down electron transport chain
• Pumps H into thylakoids
• Used to make ATP out of ADP and NADPH out of NADP
• Calvin Cycle Reaction - CO2 is taken up and
  reduced to a carbohydrate
• Reduction requires ATP and NADPH

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Light Reactions

• Light reactions consist of two electron pathways
• Noncyclic electron pathway
• Cyclic electron pathway
• Capture light energy with photosystems
• Pigment complex helps collect solar energy like
  an antenna
• Occur in the thylakoid membranes
• Both pathways produce ATP, but only the noncyclic
  pathway also produces NADPH

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Noncyclic Electron Pathway

• Electrons flow from water to NADPH
• Uses 2 photosystems, PS I and PS II
• Photosystem consists of pigment complex and
  electron acceptor molecules in the thylakoid
  membrane
• Pigment complex helps gather solar energy
• Reaction center pair of chlorophyll a molecules
  where electrons are concentrated

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Noncyclic Electron Pathway

• PS II captures light energy
• Causes an electron to be ejected from the
  reaction center (chlorophyll a)
• Electron travels down electron transport chain to
  PS I
• Replaced with an electron from water
• Which causes H to concentrate in thylakoid
  chambers
• Resulting in ATP production

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Noncyclic Electron Pathway

• PS I captures light energy and ejects an electron
• Transferred permanently to a molecule of NADP
• Resulting in NADPH production

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Cyclic Electron Pathway

• Cyclic pathway involves PS I
• PS I pigment complex absorbs solar energy and
  passes it from one pigment to another until it is
  concentrated in a reaction center
• Pathway only results in ATP production

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Thylakoid Organization

• PS II
• Pigment complex and electron-acceptors
• Adjacent to an enzyme that oxidizes water
• Oxygen is released as a gas
• Electron Transport System
• Consists of cytochrome complexes
• Carries electrons between PS II and PS I
• Also pump H from the stroma into thylakoid space

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Thylakoid Organization

• PS I
• Pigment complex and electron acceptors
• Adjacent to enzyme that reduces NADP to NADPH
• ATP Synthase Complex
• Has a channel for H flow
• Which drives ATP synthase to join ADP and Pi

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ATP Production

• Thylakoid space acts as a reservoir for hydrogen
  ions (H)
• Each time water is oxidized, two H remain in the
  thylakoid space
• Electrons yield energy
• Used to pump H across thylakoid membrane
• Move from stroma into the thylakoid space
• Flow of H back across thylakoid membrane
• Energizes ATP synthase
• Enzymatically produces ATP from ADP Pi
• This method of producing ATP is called
  chemiosmosis

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Calvin Cycle Reactions

• Calvin cycle is a series of reactions that
  produce carbohydrates before returning to the
  starting point
• Utilizes atmospheric CO2 to produce carbohydrates
• Known as C3 photosynthesis
• Includes the following processes
• Carbon dioxide fixation
• Carbon dioxide reduction
• RuBP Regeneration

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Calvin Cycle Reactions

• Carbon Dioxide Fixation
• CO2 is attached to RuBP resulting in a 6-carbon
  molecule which splits into two 3-carbon molecules
  (3PG)
• Rubisco (RuBP Carboxylase) is enzyme used

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Calvin Cycle Reactions

• Reduction of Carbon Dioxide

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Calvin Cycle Reactions

• Regeneration of RuBP

5 PGAL or
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Importance of Calvin Cycle

• PGAL or G3P (glyceraldehyde-3-phosphate) is the
  product of the Calvin cycle that can be converted
  to a variety of organic molecules

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C4 Photosynthesis

• In hot, dry climates, net photosynthetic rate of
  C4 plants is about 2-3 times that of C3 plants
• Stomata must close to avoid wilting
• CO2 decreases and O2 increases
• O2 starts combining with RuBP instead of CO2
• Photorespiration, avoided in C4 plants
• In C4 leaf, bundle sheath cells and mesophyll
  cells contain chloroplasts
• Mesophyll cells are arranged concentrically
  around the bundle sheath cells

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C3 vs C4
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CO2 Fixation in C3 and C4 Plants
C4 Plant
C3 Plant
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CAM Photosynthesis

• Crassulacean-Acid Metabolism
• C4 plants partition carbon fixation in space,
  while CAM partitions by time
• During the night, CAM plants fix CO2, forming C4
  molecules, which are stored in large vacuoles
• C4 molecules release CO2 to Calvin cycle when
  NADPH and ATP are available
• Water Conservation

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CAM Photosynthesis
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